Process for preparing azulenes



United States Patent PROCESS FOR PREPARING AZULENES Karl liegler andKlausHafnen'Mulheim an der Ruhr, Germany No Drawing.Applicationseptember 29, "1955, Serial No. 537,574

Claims priority, application Germany August 7,1953 Claims. (Cl. 260-666)This application is atcontinuation in part application of copen'dingapplication Serial No. 447 9l'8, filed August 4, 1954, nowabandoned,.for aprocess forthe preparation of azulenes.

The invention of the present application relates to a process forpreparing azulenes.

The known processes for the preparation of ,the hydrocarbon'known asazulene of thefollowing formula:

and its substitution products proceed .by .way of intermediate products,which contain the characteristicnbicyclicrii g system of azulene, namelya fiveemembered ring condensed with a seven-membered ring, but .stillnot the .full number of five doublebonds. For thettrans- ,formation ofthese partially hydrogenated intermediate products into the azulenesthemselves, dehydrogenation is carried out on conclusion of the hithertoknown asyntheses, for example by heating with palladium to a hightemperature or in some other manner. The yields in such cases areuniformly unsatisfactory, and frequently even decidedly poor. The finalyields of .azulenes, cal- 'culated on the initial substances requiredfor the synthesis of'the first initial StageS perhaps amount to a fewpercent and frequently only fractions of one percent.

The present invention provides a'process which renders itpossible toprepare azulenes directly, without dehydrogenation being necessary. Theyields are much better than with the prior known methods; for exarnple ayield of 60% and more was produced in the decisive final phase of thesynthesis of azulene (C10Hs) according to the present invention.

It has been found that the compounds of the following vigeneral FormulaI thatthe preparation of thetazulenes becomes possible ,on

a technical scale. his in general advisable to heat the pound. may alsobe form. II:

2,766,304 Patented Oct. 9, 1956 starting materialsto temperatures ofabout 300 C. the preferred range of temperatures being -300 C. Ascatalytic mediarthere may be used small amounts of alkali,.acids,,metals in the form of powders or salts but preferably .highboiling solvents or diluents of basic character (such as benzidine or4,4'-diamino-dipheny1- methane. Any solvent ordiluent used in theprocess according to the invention must not interfere with the ringclosing reaction and be inert with regard to the re- .Jactingsubstances. The starting materialsmay be heated as .such i. e. llltht)absence .of solvents or diluents or the heating maybe carried outin thepresence of solvents or diluents.

The hydrogen atoms otherwise present, that is to say .except thehydrogen atom designated by Z and standing .in one of theortho-positions to the side-chain in the .molecule of theinitialtsubstances, may be replaced byaany jdesiredmonovalentsubstituents, but the substituentsmust be so.selecte.d that they do notdisturb the ring-closing reaction. .It may be preferred in this casetouse monovalent neutral substituents, such as alkyl, aralkyl or arylradicals, especially as it is possible with these substituents for.final products havingpharmaceutically valuable properties .to beobtainedin .a :manner known per se. .Benzocyclopentadienes, i. e..indenes :and other ring systems of this type, may serve as substitutedcyclopentadiene sfor.thepurposes of the invention.

The initial substances .ofFormula-I may be present in manycases in .atautomeric form. For example, if Y in .Formula vIstands for the hydroxygroup, this compresent in the following .tautomeric ,cording to,theinvention in the same manner as the compounds of FormulaI.

The products of the tautomencform II form acetals very readily.Theselacetals may likewise be introduced asinitial substances for theprocess of the invention. The reaction when using acetals as initialproducts probably proceeds in that initially 1 mol of alcohol is splitoff and the corresponding vinyl .ether is formed. The latteris then onceagain a product in .the sense of the he general Formula I. In this casethe process proceeds according to the following equation:

on 4 H The acetals are therefore to be considered as equivalents of theproducts of the general Formula I for the purposes of the presentinvention.

As mentioned before the closing of the ring is greatly accelerated byheating the starting materials to an elevated temperature between about100 and about 300 C. The closing of the ring is shown by the blue colourwhich appears when the fulvene serving as the intermediate product isheated. A good result is obtained by heating the starting materials invacuo or by distillation with superheated steam. It is also possible touse a relatively high temperature and catalytically acting media incombination.

The yield of azulenes is frequently improved if the operation is carriedout in the presence of high boiling inert solvents or diluents. Theclosing of the ring appears to be facilitated thereby. For example,instead of the initial substances being heated as such, these substancesmay be boiled in a high-boiling solvent and the azulenes which areformed may be drawn oft either with the vapours of the solventsthemselves or by means of vapours of other solvents which are passedthrough the reaction mixture or also by a stream of gas. In these cases,the azulene can be isolated from the distillate and separated frompossible accompanying substances by known processes for example byshaking with acids or by way of the trinitrobenzolates.

Since the azulenes which are formed by the reaction are sensitivesubstances, that are capable of undergoing undesirable side reactionswhen subjected for instance for a longer time to higher temperatures, itis expedient that the azulenes should be subjected for only the shortestpossible time to the reaction conditions serving to effect thering-closing reaction. It is therefore advisable to re- "-move theazulenes from the reaction mixture as soon as they have formed. A shortstay of the azulenes in the reaction chamber may be achieved by carryingout the ring-closing reaction and the removal of the formed azulenesfrom the reaction chamber as a continuous process. The combination of ashort stay of the azulenes in the reaction chamber and the working inthe presence of solvents or diluents has proved particularly expedient.This embodiment is illustrated in some of the examples given furtherbelow.

The initial substances of the aforementioned Formula I may be obtainedby condensing a cyclopentadiene, substit'uted if desired as mentionedabove, with a compound of the following formula:

wherein Y has the same significance as in Formula I, to form thecorresponding fulvene. The formation of the fulvenes may be effected inconventional manner by means of alcoholic alkali or alkali alcoholates.It may also be advisable to condense metal compounds (for example Na, K,Mg or Ca substitution products) of cyclopentadienes with a substance ofthe Formula III.

Initial products which are available in a particularly are the compoundsof the following Formula IV:

gives azulene and methyl aniline very smoothly when it convenient mannerand which are exceptionally suitable is distilled, advantageously invacuo, or also when superheated steam is passed over it.

In a similar manner, isopropyl-cyclopentadiene gives an oily product ofa deep blue colour, which may easily be purified in known manner by Wayof the molecular compound with trinitrobenzene. The light absorption ofthe isopropyl azulene, which inter alia shows strong maxima at 608, 665and 738 up. proves that substantially or exclusively l-isopropyl azuleneis present.

Instead of the compounds of Formula IV, the compounds which can bederived from glutacon dialdehyde by substitution may also be used forthe process according to the invention. Thus, the azulene syntheses arealso successful with monoanilides of glutacon dialdehyde, for examplethe p-chloroanilide of the formula described by Th. Zincke and Schreyer,Liebigs Annalen,

vol. 353, page 384 (1907). Moreover, the Th. Zincke cleavage reactionwith dinitrochlorobenzene may also be applied without particulardifficulties to [3-picoline, derivatives of methyl glutacon dialdehydebeing formed which then, after the condensation with cyclopentadiene orits homologues, give azulenes which also contain methyl in theseven-membered ring. It is obvious that instead of the Th. Zinckedinitrochlorobenzene reaction for the preparation of the analogousderivatives of glutaconic aldehyde, it is also possible to use othersimilar cleavage reactions of pyridine, for example the cleavage ofcyanogen bromide according to W. Konig, I. pr. Chem. (2), 69, (1904), orthat with chlorosulphonic acid esters according to P. Baumgarten, Ber.1924, II, 1622.

As already mentioned, it is also possible for benzocyclopentadiene, i.e. indene, to be used instead of cyclopentadiene. The benzofulveneproduced from indene and 5-N-methylanilido-pentadienal, upon beingheated under high vacuum, very readily gives the 1.2-benzazulene withthe properties described by Plattner, Fiirst, Chopin and Winteler(Helvetica Chimica, Acta, vol. 31, 501 (1948).

Example 1 18.7 g. A mol) of 1-N-methylanilido-pentadiene- 1.3-al-5,prepared according to Th. Zincke, Liebigs Annalen der Chemie, vol. 333,page 296 (1904) and Th. Zincke and W. Wiirker, Liebigs Annalen derChemie, vol. 338, pages 121127 (1905), are dissolved by heating in 50cc. abs. ethanol and poured together with 6.6 g. (14 mol) ofcyclopentadiene into a solution of 2.3 g. of sodium in 30 cc. of abs.ethanol. The mixture becomes very hot and is cooled with ice. Thereaction solution assumes a dark red colour. The crystal magmaseparating out on cooling is filtered off with suction, well Washed withstrongly diluted acetic acid and, after drying, recrystallized fromcyclehexane or ethyl alcohol. 23 g.

(98% of the theoretical yield) of 5-(N-methylanilid0)-pentadienylidene-cyclopentadiene in the form of crystals of deep redcolour are obtained, having a melting point of 111 to 112 C. 1 g. ofthis fulvene is heated under high vacuum to -200 C. Methyl aniline andazulene distil into a receiver cooled to 80 C. After about five hours,the precipitate is taken up in ether and the blue-green etherealsolution is washed several times with N/ 10 hydrochloric acid. s5 v theether is distilled off in a small column.

The azulene solution, which is washed neutral and which is now pureblue, is dried and 340 mg. (=63% of the theoretical yield) ofcrystallized azulene with a melting point of 96 to 98 C. remain asresidue.

The melting point of the trinitrobenzolate is to 166 ,C. The. fulvenemay also be split up into azulene and methyl aniline by passing over itsteam superheated to about 300 C. The azulene then floats in thedistillate in blue flakes and is isolated therefrom with ether and thenfreed from methyl aniline by hydrochloric acid, as described above. Theyield of pure azulene corresponds to 180-190 C.

"for hours at room temperature.

colour. stand for twenty-four hours at room temperature, and the'n thereaction mixture is poured intostron'gly diluted acetic acid and thedark red oily layer of l-N-methylgrease;

to the yield of a'i'ulene obtained by decomposition under high vacuum.

Example 2 A solution of 10.8 g. mol) of isopropyl cyclopentadiene(prepared from dimethyl fulvene with lithium aluminium hydride), 18.7 g.mol) of l-(N-methylanilido)-pentadiene-1.3-al-5 in 50 cc. of abs. ethylalcohol is poured into a solution of 2.3 g. of sodium mol) in cc. ofabs. ethanol and the solution, which assumes a dark red colour, is verybriefly heated. After about one hour, the solution is introduced intostrongly diluted acetic acid and the oil separating out on the surfaceis absorbed in ether. The ethereal solution is washed until neutral,dried and then the ether is distilled off. As residue, there remains adark red viscous oil, which solidifies in crystalline form upontrituration with cyclohexane.

The 5 N-methylanilido-pentadienylidene-isopropylcyclo- "pentadienerecrystallised from cyclohexane melts at 398 C.

1 g. of the dark red oil is heated under .high vacuum Isopropyl azuleneand methyl aniline are then distilled into a strongly-cooled trap(acetone and carbon dioxide). After about five to eight hours, 110further isopropyl azulene distils over. The precipitate .is absorbed inether, Washed several times with N/ 10 HCl and dried over calciumchloride after it has been washed .neutral. The ether is then distilledoff. I A blue viscous oil is left, the trinitrobenzolate of which has amelting point of 112 to 113 C. Yield of isopropyl azulene: 350 mg.=56%of the theoretical yield.

Example 3 18.7 g. i mol) of l N-methyl-anilidmpentadiene-1.3- al-S aredissolved by heating in cc. of absolute ethanol "and poured togetherwith 8 g. A mol) of methyl cyclopentadiene into a solution of 2. 3 g. ofsodium in 30 cc. of absolute ethanol. The solution instantly assumes adark red colour. It is gently heated and then left to stand The reactionmixture is then introduced into strongly diluted acetic acid and 5N-methyl-anilido-pentadienylidene-methylcyclopentadiene separating outon the surface as an oil is absorbed in ether. The ethereal solution isdried and a viscous oily residue remains after filtering off the etherwith suction. 1 g. of this residue is heated under high vacuum to 200 C.N-methyl aniline and methyl azulene are distilled into a preliminarydeep-cooled receiver. After completing the distillation the precipitateis taken up in ether, the :N-methyl aniline is removed by -N/ 10 hydro-'chloric acid .from the ethereal solution, the latter is washed untilneutral and the now pure blue ethereal methyl azulene solution is dried.After distilling off the ether in a small column, 400 mg. (=67% of thetheoretical yield) of methyl azulene are left behind as a blue oil. Thewell crystallized trinitrobenzolate 'melts at :157-159 C. The lightabsorption of the methyl azulene shows inter alia strong maxima at 608,670 and 740 .1 and is evidence, as is also the melting point of thetrinitrobenzolate, that substantially or exclusively l-methyl azulene ispresent.

Example 4 A solution of 11.6 g. 4 mol) of freshly distilled J'indene and18.7 g. A mol) of S-N-methyl-anilidopentadienal in 30 cc. of absoluteethyl alcohol is added to a solution of 2.3 g. of sodum in 30 cc. ofabsolute "ethyl alcohol. The mixture instantly assumes a deepr'ed Afterbrief heating the mixture is allowed to anilido-pentadienylidene-indeneis taken up in ether.

After drying the ethereal solution and distilling off the ether, aviscous oil which is deep red in colour is .left. :portion "of i' thisoil is heated under high vacuum to 6 250- 3O0 C. N-methyl aniline and1.2-benza'zulene are distilled into a preliminary deep-cooled receiver.After the distillation has been completed, the precipitate is taken upin ether, the greenish-blue solution in other is shaken several timeswith N/ 10 hydrochloric acid and dried after it has been washed neutral.With the ether being distilled oif, there are obtained crystals whichare dark blue to violet in colour and have a melting point of 175 C. Thetrinitrobenzoate has a melting point of 154 to 155 C.

Example 5 93 g. (1 mol) of ,B-picoline (98%) are added together with 107g. (1 mol) of N-methyl aniline, while cooling with ice, to a solution of116 g. of cyanogen bromide in 300 cc. of :ether (prepared according toF. Baum, Berichte der Deutschen Chemischen Gesellschaft 41 (1908) 523).After a violent reaction, an orange-red crystal magma is precipitated,which is immediately filtered with suction and recrystallized fromethanol. There are obtained 191g. of the theoretical yield) of1-N-methyl-anilido-methyl-pentadiene=l.3-al-5-cyanarnide in orange-redneedles with a melting point of 146 to 147 C.

22.5 g. A mol) of this cleavage product of ,G-picoline are dissolved incc. of methanol and shaken for about fifteen minutes at room temperaturewith 50 cc. of 20% aqueous caustic soda solution. The orange-red colourthen changes to yellow. The solution is then diluted with water,neutralised with dilute acetic acid and the yellowish-brown oilseparated out is taken up in ether. After drying the ethereal phase anddrawing off the ether, there remains a brown crystal magma which,recrystallised from hexane, jyieldsl-N-methyl-anilidomethyl-pentadiene-1.3-al-(5) in the form of yellowcrystals with a melting point of 93 to 95 C. and in a yield of 148 g.(=74% of the theoretical).

10 g. (V mol) of this aldehyde are introduced with 3.3 g. ,5 mol) of.cyclopentadiene into a solution of 1.5 g. of sodium in 20 cc. ofabsolute ethanol. The solution immediately becomes dark red in colourand heats up slightly. After two hours, the reaction mixture isintroduced into strongly diluted acetic acid and the 1- N methyl-anilidomethyl-pentadienylidene-cyelopentadiene separating out as a red oil istaken up in ether. After drying the ethereal solution and distilling offthe ether, the residue is heated underhigh vacuum to .150 to C. Theazulene formed distils over as a blue oil together with N-methyl anilineinto a deep-cooled receiver. After completing the distillation, theprecipitate is absorbed in ether and the N-methyl-aniline is removedwith N/ 10 hydrochloric acid. The ethereal methyl azulene solution,which is pure blue in colour, is dried. After removing the ether, an oilwhich is deep blue'in colour remains. The said oil has the properties"and physical constants described in literature for S-methylazulene.

Example 6 One part by weight of solid crystalline5-(n-methylanilido)-pentadienylidene-cyclopentadiene are thoroughlymixed with 10 parts by weight of crystalline benzidine being as far aspossible purifiedso as to remove any parts of benzidine salts. Themixture is pressed intothe form ofpastilles, preferably on a tabletmachine.

An iron vessel having a "capacity of about 510 liters is charged with 1kg. of benzidine and heatedtoaboutl300" C. Water vapors superheated toabout 300 C. {are introduced into the vessel and are leaving the vesselby way of a wide descending condenser. The iron vessel is provided atits top with a long vertical tube carrying at its upper end a mechanismfor introducing the pastilles into the vessel during the processing. 50g. of the pastilles are charged each time into the vessel. Thedistillate going When after some rninutes no:

.tilling over the next charge of 50 g. of the pastilles is introducedinto the vessel until the mixture is worked The process is conducted insuch a way that the volume of benzidine in the reaction vessel remainsas far as possible constant. This may be achieved for instance byintroducing continuously about the same quantity of benzidine as iscarried off. At the end of the experiment the charge contained in thevessel is distilled over with the superheated steam as far as possible.The greatest part of the water is removed from the combined distillatesby suction and the solid blue coloured mass of crystals is subjected toa distillation with saturated water vapors of forms pure crystallizedazulene the yield amounting to 0.35-0.40 g. azulene per g. of fulvenecharged as starting product i. e. 65-70% of the theoretical yield. Theyield is independent from the size of the charge in question.

Example 7 A mixture of 1 g. ofS-(N-methyl-anilido)-pentadieniylidine-cyclopentadiene and of 50 g.benzidine is subjected to a distillation with water vapors superheatedto 300 C. The azulene formed thereby and part of the benzidine aredistilling over with the water vapors. The

distillate is extracted with hexane. The benzidine is removed from theextract by washing the extract with dilute hydrochloric acid. Thesolution of the azulene in hexane thus obtained is dried. The hexane isdistilled off in a small column. There are obtained as residue 380 mg.crystallized azulene i. e. 70% of the theoretical yield. Example 8 l g.of S-(N-methylanilido)-pentadienylidene-cyclo- -pentadiene arethoroughly mixed with 20 g. of copper powder and are then superheated to100 C. under a high vacuum. Azulene and methylanilin are distilled intoa deep cooled receiver. After 34 hours the contents of the receiver aredissolved in hexane. The bluegreen solution in hexane is washed to aneutral reaction and dried. The hexane is distilled off in a smallcolumn. 360 mg. (i. c. 66.7% of the theoretical yield) of crystallizedazulene are obtained melting at 96-98 C.

Example 9 2.2 g. i mol) sec. undecyl-cyclopentadieue (prepared by partlyhydrogenating with lithium aluminium hydride the fulvene obtained frommethylnonylketon and cyclopentadiene) and 1.87 g. 4 mol) ofl-N-methylanilido-pentadien-1.3-al-5 are dissolved in cc. abs.

- ethanol and poured into a solution of 0.3 g. sodium in 5 cc. abs.ethanol. The reaction mixture assumes a slightly elevated temperatureand a deep dark red colour. After 2 hours the alcohol is distilled offin vacuo. The residue forming a deep red coloured viscous oil is mixedwith 50 -g. benzidine and subjected to a distillation with water 'vaporssuperheated to 300 C. The azulene formed by the reaction is distillingover with the water vapors.

After-the distillation the azulene is extracted from the distillate withhexane. The benzidine is removed from the hexane solution by washingwith dilute hydrochloric acid. The deep blue coloured hexane solution isdried and the hexane is distilled ofi. 1.4 g. (50% of the theoretiicalyield) of a deep blue oil remain as residue represent- "ing according tothe elementary analysis, the molecular weight and the absorptionspectrum the l-scc. undecyL azulene.

Example 10 A solution of 2.32 g. (1/100 mol) ofbenzhydril-cyclopentadiene (obtained by hydrogenating partly withlithium aluminium hydride the fulvene obtainable from benzophenone andcyclopentadiene) and 1.87 g. of l-N- methylanilido-pentadien-1.3-al-5 in10 cc. of abs. ethanol are poured into a solution of 0.3 g. of sodium in5 cc. of abs. ethanol. The reaction mixture is heated slightly for ashort time, whereby it assumes a dark red colour. After about 3 hoursthe ethanol is sucked off. The residue is mixed with 50 g. of benzidineand subjected to a distillation with water vapors superheated to about300 C. Azulene is going over with the water vapors and is extracted fromthe distillate with ether. Benzidine is re moved from the extract bywashing with dilute hydrochloric acid. The dep blue coloured solution isdried and the ether is distilled 013?. The residue forms blue crystalsmelting at -91 C. and having a molecular weight of 289 (calc. 294) andrepresents the l-benzhydrilazulene. The yield amounts to 1.3 g.

Example 11 A solution of 1.79 g. (1/ mol) of 2-diethylaminoisopropyl-cyclopentadiene (obtainable by partly hydrogenating withlithium aluminium hydride the fulvene obtaned from diethylamino-acetonand cyclopentadiene) and of 1.87 g. (1/100 mol) ofl-N-methyl-anilido-pentadien-1.3-al-5 in 10 cc. of abs. ethanol ispoured into a solution of 0.3 g. sodium in 5 cc. of abs. ethanol. Thereaction solution assumes a deep red colour and is freed from ethanolunder vacum after 1 hour. The remaining red oil is mixed with 50 g. ofbenzidine and subjected to a distillation with water vapors superheatedto about 300 C. The azulene formed by the reaction is going over withthe water vapors and is extracted from the distillate with hexane. Thedeep blue coloured solution in hexane is dried and the hexane isdistilled off. 1.3 g. of 1-(2-diethylamino-isopropyl)-azulene remain asa deep blue coloured oil being easiy soluble in dilute hydrochloricacid.

What we claim is:

1. Process for preparing azulenes which comprises subjecting a member ofthe group consisting of the compounds of the general formula CH-CHwherein Z stands for hydrogen, Y represents a member of the groupconsisting of the halogens, the hydroxygroup, the alkoxygroups and thesubstituted and unsubstituted aminogroups and the hydrogen atomsotherwise present in the molecule may be substituted by monovalentradicals, and of the tautomers of the said compounds to a temperature ofabout 100300 C.

2. Process for preparing azulenes which comprises subjecting a member ofthe group consisting of the compounds of the general formula OBI-CH.

z on

jecting a member of the group consisting of the compounds of the generalformula OPP-CH z on Y-C H 04 wherein Z stands for hydrogen, Y representsa member of the group consisting of the halogens, the hydroxygroup, thealkoxygroups and the substituted and unsubstituted aminogroups and thehydrogen atoms otherwise present in the molecule may be substituted bymonovalent radicals, and of the tautomers of the said compounds to atemperature of about 100-300 C. and removing from the reaction zone theazulene formed by the reaction immediately after its formation.

5. Process for preparing azulenes which comprises subjecting a member ofthe group consisting of the compounds of the general formula wherein Zstands for hydrogen, Y represents a member of the group consisting ofthe halogens, the hydroxygroup, the alkoxygroups and the substituted andunsubstituted aminogroups and the hydrogen atoms otherwise present inthe molecule may be substituted by monovalent radicals, and of thetautomers of the said compounds to a temperature of about IOU-300 C. inthe presence of a high boiling inert solvent and removing from thereaction zone the azulene formed by the reaction immediately after itsformation.

6. Process for preparing azulenes which comprises subjecting a member ofthe group consisting of the compounds of the general formula wherein Zstands for hydrogen, Y represents a member of the group consisting ofthe halogens, the hydroxygroup, the alkoxygroups and the substituted andunsubstituted aminogroups and the hydrogen atoms otherwise present inthe molecule may be substituted by monovalent radicals, and of thetautomers of the said compounds to a temperature of about 100-300" C. inthe presence of a high boiling inert solvent of basic character andremoving from the reaction zone the azulene formed by the reactionimmediately after its formation.

7. Process for preparing azulenes which comprises mixing a member of thegroup consisting of the compounds of the general formula CHCH Z OH

wherein Z stands for hydrogen, Y represents a member of the groupconsisting of the halogens, the h'ydroxygroup, the alkoxygroups and thesubstituted and unsubstituted aminogroups and the hydrogen atomsotherwise present in the molecule may be substituted by monovalentradicals, and of the tautomers thereof with a high boiling inert solventof basic character and subjecting the said mixture to a distillationwith water vapors superheated to about -300 C.

8. Process for preparing azulenes which comprises mixing a member of thegroup consisting of the compounds of the general formula CHCH Z CHwherein Z stands for hydrogen, Y represents a member of the groupconsisting of the halogens, the hydroxygroup, the alkoxygroups and thesubstituted and unsubstituted aminogroups and the hydrogen atoms.otherwise present in the molecule may be substituted by monovalentradicals, and of the tautomers of the said compounds to a temperature ofabout 100-300 C. in the presence of a high boiling inert solvent ofbasic character and subjecting the said mixture to a distillation undera high vacuum at a temperature of about -300 C.

9. Process for preparing azulenes which comprises condensing =acyclopentadiene, which is substituted in at least one ontho-position tothe CHz-group by hydrogen, with a compound of the general formulawherein Y represents a member of the group consisting of the halogens,the hydroxygroup, the alkoxygroups and the substituted and unsubstitutedaminogroups and the hydrogen atoms being present in the molecule may besubstituted by monovalent radicals, and subjecting thepentadienylidene-cyclopentadiene-derivative thus obtained to atemperature of about 100-300" C.

10. Process for preparing azulenes which comprises condensing acyclopentadiene, which is substituted in at least one ortho-position tothe CHz-group by hydrogen, with a compound of the general formulawherein Y represents a member of the group consisting of the halogens,the hydroxygroup, the alkoxygroups and the substituted and unsubstitutedaminogroups and the hydrogen atoms being present in the molecule may besubstituted by monovalent radicals, and subjecting thepentadienylidene-cyclopentadiene-derivative thus obtained to atemperature of about 100-300 C. in the presence of a high boiling inertsolvent of basic character.

References Cited in the file of this patent UNITED STATES PATENTS2,629,750 Rath Feb. 24, 1953 OTHER REFERENCES Gordon: Chemical Reviews,vol. 50, No. 1 (February 1952), pages 127-200.

1. PROCESS FOR PREPARING AZULENES WHICH COMPRISES SUBJECTING A MEMBER OFTHE GROUP CONSISTING OF THE COMPOUNDS OF THE GENERAL FORMULA